When interest rates went skyward in November 1979 and February 1980, businesses borrowing extremely large sums were paying generally higher interest than the average rate on the smallest loans, according to a study by the Federal Reserve Board.

The study found that banks have dramatically increased the number of very large loans that they make at rates below prime -- the rate generally reserved for the banks' best customers. Much of that increase has been to allow banks to compete with the commercial paper market, according to Federal Reserve Board Chairman Paul A. Volcker.

Even so, "smaller borrowers, on average, acquired credit more cheaply in late 1979 and early 1980 than large borrowers," reversing a longstanding trend, according to the study.

The report was compiled in response to Congressional criticism that major banks were slipping below-prime rate loans to their largest customers to offset the impact of rapidly rising interest rates. Those loans are generally loans of extremely short maturities, Volcker noted.

"The study does not clearly indicate a more favored position for large borrowers in the market for credit at maturities that are more typically relied upon by both large and small borrowers," he wrote Rep. Henry S. Reuss (D-Wisc.), chairman of the House Banking, Finance and Urban Affairs committee.

Volcker said that data supplied by Reuss "do not themselves appear to justify sweeping charges of discrimination against particular groups of borrowers."

Reuss said that bank loans at below-prime rates rose from 8.8 percent in 1977 to 16.1 percent in 1978, to 32.6 percent in 1979 and to "an astounding 58.8 percent of all new loans at 48 banks recently surveyed by the Federal Reserve."